The art of lithographic printing is based upon the immiscibility of oil and water, wherein the oily material or ink is preferentially retained by the image area and the water or fountain solution is preferentially retained by the non-image area. When a suitably prepared surface is moistened with water and an ink is then applied, the background or non-image area retains the water and repels the ink while the image area accepts the ink and repels the water. The ink on the image area is then transferred to the surface of a material upon which the image is to be reproduced, such as paper, cloth and the like. Commonly the ink is transferred to an intermediate material called the blanket, which in turn transfers the ink to the surface of the material upon which the image is to be reproduced.
Aluminum has been used for many years as a support for lithographic printing plates. In order to prepare the aluminum for such use, it is typical to subject it to both a graining process and a subsequent anodizing process. The graining process serves to improve the adhesion of the subsequently applied radiation-sensitive coating and to enhance the water-receptive characteristics of the background areas of the printing plate.
Lithographic printing plates of the type described hereinabove are usually developed with a developing solution after being imagewise exposed. The developing solution, which is used to remove the non-image areas of the imaging layer and thereby reveal the underlying porous hydrophilic support, is typically an aqueous alkaline solution and frequently includes a substantial amount of organic solvent. The need to use and dispose of substantial quantities of alkaline developing solution has long been a matter of considerable concern in the printing art.
Efforts have been made for many years to manufacture a printing plate that does not require development with an alkaline developing solution. Examples of the many references relating to such efforts include, among others: U.S. Pat. No. 3,506,779 (Brown et al), U.S. Pat. No. 3,549,733 (Caddell), U.S. Pat. No. 3,574,657 (Burnett), U.S. Pat. No. 3,793,033 (Mukherjee), U.S. Pat. No. 3,832,948 (Barker), U.S. Pat. No. 3,945,318 (Landsman), U.S. Pat. No. 3,962,513 (Eames), U.S. Pat. No. 3,964,389 (Peterson), U.S. Pat. No. 4,034,183 (Uhlig), U.S. Pat. No. 4,054,094 (Caddell et al), U.S. Pat. No. 4,081,572 (Pacansky), U.S. Pat. No. 4,334,006 (Kitajima et al), U.S. Pat. No. 4,693,958 (Schwartz et al), U.S. Pat. No. 4,731,317 (Fromson et al), U.S. Pat. No. 5,238,778 (Hirai et al), U.S. Pat. No. 5,353,705 (Lewis et al), U.S. Pat. No. 5,385,092 (Lewis et al), U.S. Pat. No. 5,395,729 (Reardon et al), EP-A-0 001 068, and EP-A-0 573 091.
Lithographic printing plates designed to eliminate the need for a developing solution which have been proposed heretofore have suffered from one or more disadvantages that have limited their usefulness. For example, they have lacked a sufficient degree of discrimination between oleophilic image areas and hydrophilic non-image areas with the result that image quality on printing is poor. In addition, they have had oleophilic image areas which are not sufficiently durable to permit long printing runs, they have had hydrophilic non-image areas that are easily scratched and worm, or they have been unduly complex and costly by virtue of the need to coat multiple layers on the support.
Ceramic printing members, including printing cylinders are known. U.S. Pat. No. 5,293,817 (Nussel et al), for example, describes porous ceramic printing cylinders having a printing surface prepared from zirconium oxide, aluminum oxide, aluminum-magnesium silicate, magnesium silicate or silicon carbide.
It has also been discovered that ceramic alloys of zirconium oxide and a secondary oxide that is MgO, CaO, Y.sub.2 O.sub.3, Sc.sub.2 O.sub.3 or a rare earth oxide are highly useful printing members, as described for example, in EP-A-0 769 372 and some of the copending and commonly assigned US applications noted above.
A zirconia ceramic having a stoichiometric composition is hydrophilic in nature. Transforming the zirconia ceramic, such as during thermal imaging, into a substoichiometric composition, renders the ceramic more oleophilic. The total surface energy change from such transformations is about 6 or 7 dynes/cm, which is sufficient to create a good image. However, image quality could be improved substantially if the surface energy differential between the imaged and non-imaged areas could be made even larger.
While the zirconia ceramic imaging members described above are highly useful, and have a number of advantages over conventional materials, there is a need to provide ceramic imaging members having a greater surface energy differential between imaged and non-imaged areas on the printing surface.